As shown in FIG. 1(a), the RACH comprises a preamble signal 601, a preamble signal 602, and a message signal 603. The preamble signals 601 and 602 are used as signals for conveying a request for authorization of message signal transmission from the mobile station to the base station. The message signal 603 is for use in a practical information transmission. The base station calculates a delay profile from the preamble signal 601 to perform path detection to find a path exceeding a predetermined threshold (power level). Unless the base station is ready to receive a message even if a path is detected as a result of the path detection, the base station transmits an NACK signal 604 indicating a negative acknowledgement (refusal to authorize the message transmission) to the mobile station as shown in FIG. 1(b). The mobile station that has received the NACK signal transmits the preamble signal 602 again and the base station performs path detection. If the base station is then ready to receive the message, the base station transmits an ACK signal 605 indicating an acknowledgment (authorization of the message transmission) to the mobile station. The mobile station that has received the ACK signal 605 effects control so as to transmit the message signal 603. For details of preamble (PRACH), NACK, ACK (MCH) and the like, refer to Section 5.2.2.1 “Physical Random Access Channel (PRACH)” of TS25.211 5.4.0, Section 5.3.3.7 “Acquisition Indicator Channel (AICH)” of TS25.211 5.4.0, or the like of 3GPP (3rd Generation Partnership Project 1; W-CDMA specifications). Furthermore, for details of transmission intervals of PRACH and AICH, refer to Section 7.3 “PRACH/AICH timing relation” of TS25.211 5.4.0. For information about PRACH and AICH protocol and transmission power controls, refer to Chapter 6 “Random access procedure” of TS25.214 5.5.0. For information about code sequences forming a preamble signal, refer to Section 4.3.3 “PRACH preamble codes” of TS25.213 5.3.0. The URL of the 3GPP specifications is “http://www.3gpp.org/ftp/Specs/.”
The message signal loaded with data need be received with fewer errors than those of the preamble signal in the base station.
As well known, for example, a rake receiver is used as an error reduction technology in CDMA. A pseudo-noise (PN) sequence for use in a spread spectrum communication causes a peak when code timings match. Upon receiving a direct-sequence spread-spectrum signal (DS signal) under a multipath environment, a correlation peak corresponding to each of the multipath can be obtained. In other words, in the direct-sequence (DS) spread-spectrum communication, the multipath can be separated temporally, thus enabling a path diversity reception in which signals having passed through the paths are individually despread and recombined. A receiver in a rake system (referred to as “rake receiver”) executing the path diversity reception performs maximum ratio combining of the paths by applying an in-phase step to outputs (branches) of correlators (referred to as “rake fingers”) that perform despreading, weighting the branch signals according to signal levels, and summing up the branch signals. The rake receiver typically comprises N correlators 701 to 70N, for example, as shown in FIG. 2. The correlators 701 to 70N perform despreading operations of input signals using a spreading code sequence (PN sequence). The rake receiver performs the maximum ratio combining through combining the results of multiplying outputs of the correlators by weighting factors 711 to 71N according to the signal levels with a combiner (adder) 720 and performs a path diversity in the maximum ratio combining, in which signal powers dispersed due to a delay dispersion of a radio transmission channel is brought together. Note that path delay information (delays #1 to #N) set for the correlators 701 to 70N is requited for the rake reception. Therefore, the correlators 701 to 70N perform an operation of correlation between the input signals and the spreading code sequence on the basis of the set delays #1 to #N. For details of the rake receiver, refer to, for example, Japanese Patent Laid-Open Publication No. 2000-232430.
To obtain delay information, obtain a delay profile at receiving a message signal and detect a position of peak power in the delay profile, in other words, a path position.
This method, however, has a problem of a high processing load though it enables accurate path detection. Furthermore, it has another problem of a delay that occurs since a rake reception cannot be started until the path detection is completed.
Therefore, conventionally there has been provided a method of resolving these problems by utilizing features of RACH. The conventional method is to set a path position detected by using a preamble signal for a rake receiver as path information of a message signal directly. According to the method, a processing load is relatively reduced at receiving the message signal following the preamble signal and a base station has less processing delay.
In general, a threshold of a signal-to-noise ratio for use in preamble signal detection, in other words, a preamble threshold is set to a relatively high value so as to prevent erroneous detection caused by noise or interference effects.
Therefore, the number of paths determined to be detected is apt to be low at preamble signal detection. Even if these paths were set directly for the rake receiver (for example, if the number of paths is far lower than that of rake fingers), an enough gain is not achieved, thus failing to reach a substantial improvement of characteristics.
In addition, if there is a large time difference between a preamble signal transmission and a message signal transmission, a change may occur in a propagation environment, by which it is very possible that a path detected at a preamble signal reception in the base station has been changed at a message signal reception.
Thereafter, if a rake reception of the message signal is started after path setting without taking into consideration the above fluctuation, an enough gain is not achieved, thereby causing frequent errors in received data.